The use of glass in the automotive, architectual and bar code scanner manufacturing industries is well known. Characteristics of glass which make it a popular material in these industries are its transparency, low cost, high strength and ability to act as a substrate. However, glass is not a particularly hard material and it frequently abrades and scratches in the above mentioned applications.
Abrasions and scratches are distinguishable. An abrasion is defined as "a wearing, grinding, or rubbing away by friction." Webster's Ninth New Collegiate Dictionary 46 (9.sup.th ed. 1988). A scratch is defined as a "scrape or dig." Id. at 1054. Furthermore, measuring materials' susceptibility to scratching is measured in a different manner than their susceptibility to abrasion. Scratch susceptibility is tested by a person skilled in the art drawing a fresh nail across the material. Abrasion susceptibility is tested by a Taber abrader, pencil erasers or ordinary abrasive cleaners applied to the material.
In the past, coatings have been used to enhance the abrasion and scratch resistance of glass. U.S. Pat. No. 5,637,353 reveals one method of producing an abrasion and wear resistant coated substrate by Chemical Vapor Deposition (CVD). The '353 patent teaches the use of a substrate, one or more intermediate layers and a top, low friction layer such as a diamond-like carbon (DLC) material. One or more optically transparent intermediate layers capable of forming a strong chemical bond to the substrate and the DLC layer is deposited onto the substrate. Each intermediate layer has a thickness of about 1 to 20 microns and has a hardness greater than that of the substrate. After the desired number of layers are deposited, a low friction DLC layer is deposited onto the coated substrate. Tin oxide is one of dozens of substances disclosed as a suitable composite layer. The final step is cooling the coated substrate to room temperature by means of passing inert gas over the substrate.
DLC films are known in the art for their abrasion and corrosion resistance and their excellent optical properties. However, the abrasion resistance of a DLC film is a function of its ability to adhere to the parent substrate. The improved abrasion resistance of DLC coatings is only available if the adherence to the substrate is excellent and this is difficult to achieve.
U.S. Pat. No. 5,635,245 teaches the same process as U.S. Pat. No. 5,637,353, but specifies several alternative intermediate layers. One intermediate layer consists of silicon oxide or aluminum oxide under a layer of material devoid of alkali atoms and fluorine. An intermediate layer consisting solely of silicon oxynitride is also taught. Alternatively, an intermediate layer consisting of silicon oxynitride combined with a layer of silicon oxide is disclosed. The final intermediate layer taught is a silicon oxynitride layer with a layer of aluminum oxide. Both this patent and the '353 patent teach away from the use of a polymeric material for the composite layer.
U.S. Pat. No. 5,232,783, issued to Pawar et al., discloses abrasion and scratch resistant coatings for glass articles. A coating composition is provided which includes an organooxy-chainstopped organopolysilsesquioxane, a tetraorganotitianate, a siloxane-functionalized ester having the general formula (R.sup.3 O).sub.3-2n SiO.sub.n R.sup.4 OC(O)R.sup.5 C(O)OR.sup.6, wherein n is equal to 0, 1/2, or 1, R.sup.3 is independently a saturated or unsaturated monovalent hydrocarbon radical containing from 1 to about 20 carbon atoms, R.sup.4 is a divalent hydrocarbon radical containing from 1 to about 20 carbon atoms, R.sup.5 is a divalent conjugated olefinic radical having from 2 to 4 carbon atoms, and R.sup.6 is R.sup.3 or R.sup.4 SiO.sub.n (OR.sup.3).sub.3-2n ; or a cyanoorganotri-organooxysilane and an organic carboxylic acid ester; and from 0% to about 85% of a volatile diluent. Also provided is an abrasion and scratch resistant coating composition for a copolymer of the organopolysilsequixane above and the siloxane-functionalized ester previously mentioned.
Despite the wide variety of methods known to produce abrasion and scratch resistant coated glass, it would be advantageous to have a coating with a lower probability of collecting and retaining deposits from metals passed over its surface to endure either high wear environments, such as the top surface of a bar code scanner, or environments susceptible to scratching by vandals.